The use of linear accelerators for generation of either electron radiation or X-ray radiation is well known. In the case of electron radiation, a scattering foil and a dose chamber for measuring the electron radiation are arranged at an exit window of the accelerator, in the trajectory of the emitted electron beam. For X-ray radiation, a target for converting the electrons to X-rays, a flattening filter for broadening the X-ray beam, and a dose chamber, are arranged at the exit of tile accelerator. These systems are typically used by the medical community for treatment of cancer by radiotherapy.
One of the challenges inherent in radiotherapy treatment is the accurate positioning of the tumor in the radiation field. The main sources of the problem result from the fact that there is a natural motion of organs inside the body, which can range from approximately a millimeter in the case of the brain inside the skull to several centimeters for the organs in the trunk above the diaphragm. Another factor relates to changes which occur in the tumor over time as a result of successful treatment; as the tumor shrinks in volume, normal tissue which had been displaced returns to its original position within the treatment volume.
To accurately verify tumor positioning, portal films or electronic portal imager systems are commonly used in the radiation treatment verification process. In the case of portal imaging, the megavolt therapeutic X-rays emerging from the patient are used to generate images. However, these verification methods deliver images of low contrast and insufficient quality.
The problems associated with utilizing high energy X-rays produced by a megavolt electron beam are the result of interacting with matter primarily due to Compton scattering, in which the probability of interactions is proportional to the electron density. Low energy diagnostic X-rays, typically have energies of about 100 Kvp, where a significant portion of the interactions with matter are photoelectric, and the interactions are proportional to the cube of electron density. Tissue in the human body is typically of low density. As a result, the contrast achieved in low energy X-rays is far superior to that obtained with megavoltage X-rays, thereby allowing better distinctions of landmark features and the imaging of other features not perceptible with high energy X-rays.
One method taught in the prior art to utilize a low dose, low energy X-rays source in conjunction with a therapy X-ray source, is the external coupling of a low energy X-ray source to the accelerator's gantry head. This off-set attachment was discussed in a paper by P. J. Biggs et al. entitled "A Diagnostic X-ray Field Verification Device for a 10 Mv Linear Accelerator", Int. J. Radiation Oncology Biol. Phys.:ii, 635-643, 1985. This arrangement, however, has the drawback of being non-coincident with the treatment beam, thereby creating problems in applications where verification of anatomical features during a treatment sequence is advantageous, e.g. during conformal therapy.